The invention relates to the field of optical pressure measurement. It proceeds from a fiber-optic laser as claimed in the preamble of claims 1 and 12.
It is necessary in crude oil production for bore holes to be monitored with regard to pressure and temperature. In the bore hole, the liquid pressures can be up to 100 MPa (1000 bar), and the temperatures can be up to over 200xc2x0 C. Electric sensors such as, for example, piezoelectric resistors, piezoelectric elements, capacitive probes or crystal resonators, are frequently used for pressure measurement up to approximately 170xc2x0 C. Also known is the use of optical pressure sensors which are distinguished by good high-temperature capability, corrosion resistance and electromagnetic insensitivity. Examples of this are mechanical resonators which are activated optically and read out optically, elastooptic sensors, optical sensors which a pressure-sensitive diaphragm, or Fabry-Perot resonators.
Polarimetric fiber laser sensors are disclosed, for example, in the article by H. K. Kim et al., xe2x80x9cPolarimetric fiber laser sensorsxe2x80x9d, Optics Letters 18 (4), pages 317 to 319 (1993). One or more longitudinal modes are caused to lase in an Nd-doped fiber with a round core and dichroically mirrored ends which are transparent to pumped light. Birefringence is produced in the fiber by unidirectional lateral pressure, and a frequency shift is induced between the orthogonal natural polarizations of the longitudinal modes. In the outcoupled beam, the natural polarizations are brought to interference by a linear analyzer, and the resulting beat signal is detected with the aid of a photodiode. The beat frequency can be measured very easily using a frequency counter. It represents a high-precision measure of the unidirectional pressure on the fiber laser. Hydrostatic pressures can, however, not be measured in this arrangement.
A strain sensor of similar design is presented in the article by G. A. Ball et al., xe2x80x9cPolarimetric heterodyning Braggxe2x80x94grating fiberxe2x80x94laser sensorxe2x80x9d, Optics Letters 18 (22), pages 1976 to 1978. Instead of the two mirrors, two Bragg gratings written directly into the fiber core are used to bound the laser cavity. Because of the small cavity length of 2.5 cm and the weak ellipticity of the fiber core, exactly two orthogonally polarized longitudinal modes can be excited using a relatively low, effectively measurable beat frequency. It is possible to use such a fiber laser sensor to measure each variable quantity which effects variation in the length or birefringence of the laser cavity. Absolute measurements, for example of a pressure, are, however, difficult or impossible, since temperature fluctuations, changes in optical parameters owing to material fatigue and the like can arbitrarily displace the operating point, that is to say the beat frequency in the unloaded state.
In the publication by J. P. Dakin et al., xe2x80x9cCompensated polarimetric sensor using polarisationxe2x80x94maintaining fibre in a differential configurationxe2x80x9d, Electronic Letters 20 (1), pages 51 to 53 (1983), a passive fiber-optic sensor is indicated which comprises two identical segments, twisted by 90xc2x0 and spliced together, of a polarization-maintaining fiber. One segment is subjected to the measured variable, for example temperature, strain or acoustic waves, and both segments are subjected to the isotropic noise quantity, for example all-round pressure or temperature. This differential arrangement is also particularly effectively compatible with low-coherence semiconductor lasers, because the intensity noise caused by phase noise of the light source is largely compensated. The detection of the interterometric, periodic signal is, however, substantially more complicated than the detection of frequency-coded signals of active fiber laser sensors.
Serial multiplexing of passive fiber Bragg grating sensors is disclosed, for example, in U.S. Pat. No. 4,761,073. A plurality of Bragg gratings with different reflection wavelengths are written in along a sensor fiber. The strains at the locations of the Bragg gratings can be determined by measuring the shifts in the reflection wavelengths. Thermally induced grating strains can be eliminated with the aid of superimposed gratings of different reflection wavelengths. It is also known to be possible to determine the location by time-resolved measurements with the aid of a pulsed light source instead of by wavelength-selective measurements. The measuring range is limited because of the risk of fiber fracture when measuring strain with the aid of Bragg gratings. Moreover, Bragg gratings are largely unsuitable or extremely insensitive for measuring hydrostatic or isotropic pressures.
In the article entitled xe2x80x9cPerturbation Effects on Mode Propagation in highly Elliptical Core Two-Mode Fibersxe2x80x9d by S,-Y. Huang et al., it is shown that a phase shift can be produced both between the polarization modes and between the spatial modes in a polarization-maintaining double-mode fiber by means of homogeneous, all-round or radial pressure, but also by axial strain, twisting and temperature.
The object of the present invention is to specify a fiber laser sensor which is suitable for frequency-coded measurement of isotropic pressures, strains or temperatures, and is distinguished by a large measuring range, a simple design and simple multiplexing capability. This object is achieved according to the invention by means of the features of claims 1 and 12.
Specifically, the core of the invention is to arrange in the laser cavity of a fiber laser, in addition to a doped fiber acting as laser medium, a sensor fiber with a non-rotationally symmetrical structure in which all-round pressure can be used to induce birefringence and a pressure-proportional beat frequency between different polarization modes or spatial modes.
An exemplary embodiment shows the optical design of a fiber laser pressure sensor with a temperature-compensated sensor fiber which consists of two fiber segments twisted relative to one another by 90xc2x0. In differential operation, both segments are subjected to the noise quantity, for example the temperature, and only one segment is subjected to the measured variable, for example the pressure.
A further exemplary embodiment represents a serial arrangement of a plurality of fiber laser pressure sensors with different emission wavelengths, which are fed via a common pumped light source, and whose pressure-proportional beat frequencies are detected in a wavelength-selective fashion.
Other exemplary embodiments relate to pressure housings for fiber lasers, in the case of which the laser-amplifying fiber and a sensor fiber segment are positioned in a capillary or chamber under low-pressure gas or a vacuum, and a sensor fiber segment is in pressure contact with the medium to be measured.
Additional exemplary embodiments follow from combination of features essential to the invention, and from the dependent claims.
An important advantage of the fiber laser pressure sensor according to the invention consists in that the frequency-coded pressure signal renders it possible to achieve a high measuring accuracy, a large pressure measuring range up to 100 MPa, and effective calibratability to absolute pressures.
A substantial advantage of the fiber laser pressure sensor also consists in that the parameters of the amplifier fiber and sensor fiber can be optimized independently of one another. In particular, it is possible to use commercially available, erbium-doped amplifier fibers and double-mode sensor fibers with an elliptical core.
A further advantage of the fiber laser pressure sensor consists in that the temperature sensitivity is largely repressed by the differential design of a sensor fiber, and in addition the temperature can be determined from the Bragg wavelength and, as a result, the reliability of (quasi-)static pressure measurements is greatly improved.
Finally, the compact and robust design, as a result of which the fiber laser pressure sensor is outstandingly suitable for use under high pressures and temperatures and, in particular, for pressure measurement in crude oil drill holes, is very advantageous.